This application claims the priority benefit of Japan application serial No. 2001-243655, filed on Aug. 10, 2001.
1. Field of the Invention
The present invention is related to an apparatus and method for assessing the state of wear of electrode tips used in consumable electrode gas shielded arc welding.
2. Description of Related Art
In consumable electrode gas shielded arc welding, an xe2x80x9celectrode tipxe2x80x9d has a consumable electrode (hereinafter, xe2x80x9cwelding wirexe2x80x9d) inserted through it in a manner such that there is electrical contact between the wire and the tip. Electrical power is then supplied to the welding wire through the electrode tip. The wire is fed through a hole in the electrode tip, causing the hole to wear. This wear progresses over time, causing the points of contact between the wire and the electrode tip (the points through which power is supplied) to shift, which in turn results in fluctuations in the state of the electrical power supplied to the wire. This causes arc instability phenomena to occur during welding.
Therefore, to keep the electrical power supply points stable and maintain good weld quality, electrode tips must be replaced when they become too worn. In the past, this tip replacement was performed either whenever deemed necessary by an equipment operator, based on his experience in visual observation of arc instability, or periodically, based on the accumulated operating time of the tip.
An apparatus and method for automatically assessing electrode tip wear in consumable electrode gas shielded arc welding was proposed in Japanese Publication of Unexamined Patent Application (hereinafter xe2x80x9cpre-grant publication) No. H8-318372 (hereinafter, xe2x80x9cthe first background art referencexe2x80x9d). In this method, electrode tip wear was assessed by detecting welding wire feed motor load current and weld current, and comparing variations in these two currents with their respective wear assessment threshold values.
Another method and apparatus for determining when an electrode tip had reached its wear limit was proposed in Japanese Pre-grant Publication No. 2000-24779 (hereinafter, xe2x80x9csecond background art referencexe2x80x9d). In this method, variations in welding current or welding voltage were sensed, and it was deemed that the electrode tip had reached its wear limit when the variations met prescribed conditions.
In the technology of the first background art reference, a key factor in the assessment of electrode tip service life was the maximum amplitude of weld current variations over a specific time period. In reality, however, in most cases, however, the maximum amplitude was simply the difference between the high current that occurs during the short-circuit time, and the minimum current. Therefore, in this technology, although it was possible to assess welding instability caused by an electrode tip at its wear limit, it was difficult to determine the optimum tip replacement timing. In other words, the system might indicate that replacement was required for an electrode tip that was still useable.
In the method described in the second background art reference, a decision that a tip was at the end of its service life was made when the welding voltage or current met prescribed conditions. Since most of the variations in welding current and voltage occur during the short-circuit time, however, it was inevitable that tip wear assessments would be heavily influenced by the short-circuit voltage or current. As was the case for the method of the first background art reference, then, although it was possible in this method to assess welding instability caused by an electrode tip at its wear limit, it was difficult to determine the optimum timing for tip replacement. Accordingly, in this method as well, the system might indicate that replacement was required for an electrode tip with useful service life remaining.
In other words, inherent in the methods of the above first and second background art references was always the possibility that a false indication of an electrode tip at the end of its service life might be caused by welding instability, irrespective of the amount of actual tip wear.
The present invention is directed to an apparatus and method to solve this problem. It is an object of the present invention to provide an apparatus and method for assessment of electrode tip wear that can perform accurate quantitative assessment of the state of wear of an electrode tip, thereby to provide a precise assessment as to the optimum timing for electrode tip replacement. This object is achieved through the technology described below.
That is, to accomplish the above object, an apparatus for assessing electrode tip wear according to one aspect of the present invention is characterized in that, in consumable electrode gas-shielded arc welding, wherein a welding voltage is applied to an electrode tip having a wire inserted therein in electrical contact therewith, for performing arc welding by causing droplets to be transferred from the welding wire onto a workpiece to be welded, it comprises: a welding voltage detection means, or a welding current detection means, for detecting welding voltage applied between the welding wire and the workpiece, or the welding current supplied; a replacement index computation means for computing an electrode tip replacement index based on the average value of the arc-time welding voltage, or arc-time welding current, as detected by the welding voltage detection means or welding current detection means; and a wear assessment means for assessing the state of wear of the electrode tip by comparing the replacement index computed by the replacement index computation means with a reference standard value. It is to be noted here that the terms arc-time welding voltagexe2x80x9d and xe2x80x9carc-time welding currentxe2x80x9d in the above description (and in the following) refer to that portion of the welding voltage and welding current that does not occur during the short-circuit time.
In the above assessment apparatus, an electrode tip replacement index is computed by the replacement index computation means, based on the average value of the arc-time welding voltage or arc-time welding current as detected by the welding voltage or welding current detection means. The wear assessment means then compares the computed replacement index value with a reference standard value, thus to provide a precise assessment as to the state of wear of the electrode tip.
It has been shown that as the accumulated time of use of an electrode tip for arc welding (accumulated operating time) increases, and the wear of the tip progresses, the average value of the arc-time welding voltage or arc time welding current, which is used to perform assessments, changes (see FIG. 5 and FIG. 6). It is this relationship that makes quantitative assessment of electrode tip wear possible. In addition, the welding voltage/current during arc-time is extremely stable in comparison to the welding voltage/current during the short-circuit time, and is subjected to few external disturbance factors other than electrode tip wear. This makes it possible to perform highly precise, quantitative assessment of electrode tip wear, and of the optimum tip replacement timing.
Also, according to another of the present invention, a method for assessing electrode tip wear in consumable electrode gas-shielded arc welding, wherein a welding voltage is applied to an electrode tip having a wire inserted therein in electrical contact therewith, for performing arc welding by causing droplets to be transferred from the welding wire onto a workpiece to be welded, is characterized in that it comprises: a step of detecting welding voltage applied between the welding wire and the workpiece, or welding current supplied; a step of computing an electrode tip replacement index based on the quotient of the instant average arc-time welding voltage, or arc-time welding current, divided by the average arc-time welding voltage, or arc-time welding current, respectively, immediately following tip replacement; and a step of assessing the state of wear of the electrode tip by comparing the value of the replacement index computed by the replacement index computation means with a prescribed assessment threshold value.
According to the assessment method as described above, an electrode tip replacement index is computed, based on the quotient of the instant average arc-time welding voltage, or arc-time welding current, divided by the average arc-time welding voltage, or arc-time welding current, respectively, immediately following tip replacement, and an assessment is performed by comparing the value of this computed replacement index to an assessment threshold value (which is set to a value at which it has been determined in advance that electrode tip replacement will be necessary). This makes it possible to provide a precise assessment of optimum electrode tip replacement timing.